This application is a continuation of international patent application no. PCT/EP00/06914, filed Jul. 19, 2000, designating the United States of America, the entire disclosure of which in incorporated herein by reference. Priority is claimed based on Federal Republic of Germany patent application no. DE 199 33 872.8, filed Jul. 23, 1999.
The invention relates to a method of producing microbore holes and to an apparatus for producing microbore holes. A method and an apparatus for producing microbore holes of the aforementioned type are known from published European Patent Application No. EP 884,128. According to this document, a substrate is arranged on an XY stage or table which can be positioned along X and Y coordinates in the desired treatment positions, whereby the bore hole coordinates of the borings to be introduced and additional information such as bore hole diameter are provided by a computing system. In order to enable the production of bore holes with diameters of 50 xcexcm or less using a conventional CO2 laser, the laser beam is converted to a beam having a small wavelength, using a tellurium crystal. Changing the diameter of the beam or the spot is not described.
Furthermore, it is problematic that when the laser power is increased to produce larger bore hole diameters, instead of a cylindrical bore hole a conical expansion of the bore hole can occur as a result of focusing the laser beam.
In addition, a method and an apparatus for treating substrates are known from U.S. Pat. No. 5,690,846, in which errors resulting from distortions and/or faulty alignments of the substrate can be compensated for by using a special computer method. There is no reference therein to influencing the diameter of the spot.
An object of the present invention is to provide a new method of the aforementioned type so as to enable rapid and reliable production of bore holes having different diameters.
Another object of the invention is to provide a new apparatus of the aforementioned type with which bore holes having different diameters can be rapidly and reliably produced.
These and other objects are achieved in accordance with the present invention by providing a method of producing microbore holes in a multilayer substrate that is displaced below writing optics by an XY stage, wherein the writing optics generate a spot from a light beam source; the position of the light spot within a working field is changed simultaneously with substrate treating positions by a positioning unit comprising electronically controlled, movable mirrors; the position of the substrate is determined; signals corresponding to the substrate position are processed by a computer to obtain an actual position of the XY stage, and the diameter of the spot is changed by an expansion ratio determined by the computer, using variable beam expansion optics.
In accordance with another aspect of the invention, the objects are achieved by providing an apparatus for producing microbore holes in a multilayer substrate comprising writing optics for generating a light spot from a light beam source; an XY stage for moving the substrate to different treatment positions below the writing optics; the writing optics including a beam deflecting unit comprising electronically controlled, movable mirrors for changing the position of the light spot within a working field on the substrate simultaneously with the treatment positions; means for determining the position of the substrate, and a computer for processing signals corresponding to the substrate position to obtain an actual position of the XY stage, wherein the writing optics further comprise a variable beam expansion optics arranged in a light beam path between the light source the beam deflection unit; the variable bean expansion optics outputing a light beam having a diameter that is varied according to an expansion ratio determined by the computer.
In the method and apparatus of the invention, the light from a pulsed laser, for example UV light from a frequency-multiplied Nd:YAG laser or infrared light from a CO2 laser, can be used to produce bore holes in materials used in the production of electronic printed circuit boards. The parameters of the light source and the optics used, such as the laser power, pulse duration, and size of the spot, are generally known to persons skilled in the art. Treatment systems of the current art basically comprise an XY stage that positions the substrate to be treated below an optical structure that is appropriate for the optical requirements. The optical structure performs two functions. First, it produces an intense pulsed laser spot for treating the substrate at the required position. Second, it determines the position by recognizing preset substrate marks from previous production steps. This step requires an image processing system comprising an electronic camera and a suitably equipped computer system that determines the desired positional information from the camera signals.
The overall precision of the position of the bore holes in the substrate relative to the preset marks is determined by the positional precision of the XY stage system, and the precision of spot positioning of both the optical beam forming system and the optical measuring system. In modern printed circuit boards constructed from a plurality of layers of conductors and insulation materials, material distortions occur during the individual production steps, making it necessary to adapt the bore hole patterns to the individual distortion of the base substrate. This in turn requires very high precision in the measuring and positioning of the XY stage system and the beam optics.
For economic reasons it is necessary to minimize as much as possible the overall treatment time as well as the time for each bore hole to be produced. Depending on the particular application or the technology of the printed circuit boards, bore hole diameters of a few tenths of a millimeter down to 50 xcexcm should be maintained. Since the spot diameter of the laser beam is approximately 25 xcexcm for typical UV laser treatment systems, bore hole diameters that deviate from this value must be created by lining up individual treatment steps, identified hereinafter as xe2x80x9cpasses.xe2x80x9d The removal of material by multiple passes along a production line, suitably chosen and generally spiral-shaped, is referred to as xe2x80x9cnibbling.xe2x80x9d Although this method allows bore hole diameters to be produced in any size, it has the disadvantage of being very time-consuming.
Since the energy requirements per pass depend greatly on the material to be treated, an optimized treatment strategy is desirable. If sufficient energy is available for each laser pulse, the throughput is significantly increased if instead of the nibbling method a more suitable, larger spot diameter is chosen, and only one pass removes the required amount of material for the desired bore hole diameter. The basis of the novel method described here is that the spot diameter of the laser beam used for treatment may be varied within a very short time, thus producing bore images of different diameters in a single operation.